Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells with the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H.sub.2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O.sub.2), or air (a mixture of O.sub.2 and N.sub.2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, in admix with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies which comprise the catalyzed electrodes are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereof.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
It is, of course, necessary especially in vehicle applications, to periodically shutdown the system. The shutdown may be a normal shutdown as, for example, upon stopping the associated vehicle, or may comprise a rapid shutdown as, for example, when an abnormality is detected in the system that might result in degradation of the system if the system is not immediately shutdown. It is in general desirable in terms of system efficiency and system stresses to shut down the system in a normal manner. Conversely, it is imperative in terms of preventing system degradation to shutdown the system on a rapid basis when there are concerns with respect to system degradation. It is therefore imperative in a successful, efficient fuel cell system that shutdown be conducted on a normal basis wherever possible, but it is equally important that the system be shutdown on an immediate, rapid basis when conditions demand.